Method of making a traveling wave guide device



E C I v E D E D I "U G E MW G m E L KL m M MT E A m I M M F 0 D O H m M 2 Sheets-Sheet 1 Filed May 1, 1963 INVENTOR. DONALD E. MACKERROW ATTORNEY March 28, 1%67 E MACKERROW 3,310,864

METHOD OF MAKING A TRAVELLING WAVE GUIDE DEVICE 2 Sheets-Sheet 2 Filed May 1, 1963 25 CLEARANCE INVENTOR. DONALD E. MACKERROW ATTORNEY United States Fatent' 3,310,864 METHOD OF MAKING A TRAVELING WAVE GUIDE DEVICE Donald E. Mackerrow, Cupertino, Califi, assignor to Huggins Laboratories, Inc., Sunnyvale, Calif., a corporation of California Filed May 1, 1963, Ser. No. 277,360 9 Claims. (Cl. 29-1555) This invention relates to traveling wave tube devices and more particularly to means and a method for bonding a slow wave interaction circuit to the envelope of a traveling wave tube device.

Traveling wave tube devices, as well known in the art, generally comprise an evacuated elongated enclosure housing a slow wave interaction circuit along which an electromagnetic wave is propagated and an electron gun means for generating and accelerating an electron stream adjacent to the slow wave interaction circuit so that the electromagnetic wave interacts with the electron stream. The most usual type of slow wave interaction circuit is in the form of a helical wire, generally referred to simply as a helix and so designated herein.

Usually the helix is supported within the evacuated elongated envelope in such a manner that its axis is symmetric and concentric with the envelope axis and the electron beam is propagated and focused along this common axis.

Heretofore, there have been a number of ways of supporting the helix with the envelope. For example, insome devices the bore of the envelope is dimensioned for minimum clearance with the helix so that, if the ends of the helix are properly secured, the bore itself supports the helix in the desired concentric relationship against transverse motion. Such a helix support however does not secure the helix against longitudinal displacement experience under vibration or shock. Further the close proximity of the envelope, which is generally a dielectric material, all around the helix caused undesirable loading which slows down the electromagnetic wave.

Another device in common use today has an increased envelope bore and utilizes three ceramic spacer rods, typically of 0.060 inch diameter, which are glazed or bonded every 120 degrees to the helix. This method of supports, particularly when the spacer rods are bonded to the helix, support the helix against both transverse and longitudinal displacement and decreases the loading on the helix since there is much less proximity between the dielectric envelope and the helix, and the loading is substantially confined to the three peripheral points of contact with the dielectric spacer rods. Such support arrangement however results in a somewhat fragile structure and is beset with fabrication problems since great care must be exercised in bonding the rods to the helix and thereafter assembling the traveling wave tube device.

To overcome some of the difficult fabrication problems, and to maintain dielectric loading of the helix at a minimum, some manufacturers today utilize an internally fluted envelope, typically having three flutes, for holding the helix in place. To secure the helix in place, the helix is bonded or wetted to the inner peripheral surface of the flutes by heating the envelope to its softening point with the helix in place and thereafter applying pressure to the outside of the softened envelope at points radially aligned with the flutes. As a result of the application of pressure, the flutes are urged inwardly into contact with the helix and wet the same in place.

This method of locking or bonding the helix into place also has several limitations both from a fabrication and from a performance point of view. Fabrication is time consuming in that the heating and subsequent cooling of the envelope takes between one or two hours. Also, as

substantially more economical to fabricate 3,31%,864 Patented Mar. 28, 1967 a result of heating the Whole envelope, it has been found that changes in the geometry of the helix and the envelope take place which deteriorate performance. The helix, instead of being circular, is now depressed in three places causing each turn of the helix to assume a somewhat triangular perimeter. Even though the indentations or dimples caused by the inwardly moved flutes may be slight, this change of geometry nevertheless has been found to materially deteriorate the performance characteristics of the device. Furthermore, the loading on the helix is increased over that obtained with ceramic spacer rods because the envelope penetrates more deeply into the helix. It has also been found that the above-described method of locking the helix into place generates stresses on the envelope causing a camber which often seriously reduces the performance of the traveling wave tube device.

It is therefore a primary object of this invention to provide a new and novel method for rigidly supporting a helix in a long tubular envelope.

It is also an object of this invention to provide a traveling wave tube device having its helix locked in place against longitudinal and transverse motion which is than was possible heretofore.

It is a further object of this invention to provide a traveling wave tube device in which the helix is wetted to the glass envelope in such a manner that the resulting dielectric loading of the helix is a minimum.

It is another object of this invention to provide a traveling wave tube device in which the helix is locked securely in place by being bonded or wetted to one narrow longitudinal section of the inner surface of the tubular envelope and which may be constructed more economically and with less geometric distortion of the helix and the envelope than has been possible heretofore.

It is still a further object of this invention to provide a traveling wave tube device which is more economical to fabricate and which has a better performance characteristic than traveling wave tube devices heretofore made.

Briefly, the present invention utilizes a tubular envelope into Which the helix is inserted and which is hermetically sealed and evacuated to a low pressure after such insertion. A torch, which travels parallel to the longitudinal axis of the envelope, is then applied to the envelope to progressively heat a small segment of the envelope wall to its softening point. The softened segment envelope wall is urged inwardly under the pressure of the vacuum into contact with the helix where it wets the individual coils opposite the softened wall section. By carefully controlling the rate of travel or the temperature, or both, and thereby the degree of softening, any desired degree of penetration of the wall section into the helix can be obtained.

Other objects and a better understanding of the invention may be had by reference to the following description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a side view of a traveling wave tube device of one specific embodiment of this invention;

FIG. 2 is an enlarged cross sectional view taken along line 22 of FIG. 1;

FIG. 3A and 3B are enlarged cross sectional views taken along line 33 of FIG. 2 respectively showing a section of the helix before and after being locked into place; and

FIG. 4 is an illustrative diagram of a traveling torch de-v vice useful in practicing the method of the present invention.

Referring now to the drawing, and particularly to FIGS. 1, 2 and 3A thereof, there is shown a traveling wave tube device constructed in accordance with the present invention. The device comprises a helix 10 forming the slow Wave interaction circuit, and a glass tube 12 to which helix is bonded. Glass tube 12, also referred to as the tubular envelope of the traveling wave tube device, has an enlarged end portion 14- commonly referred to as the bulb which is dimensioned to accommodate an electron beam gun schematically shown at 16 for generating and accelerating an electron beam 18. Electon beam 18 is accelerated for propagation along the longitudinal axis of helix 10 and is collected at the other end of the device by a collector schematically shown at 20. Microwave energy may be conventionally coupled into and out of helix 10 by means of a pair of coupling helixes 22 and 24 respectively.

Glass tube 12, as best seen in FIG. 2, is internally fluted, to define three flutes 26, 27 and 28 angularly displaced with respect to one another by 120 degrees. Each flute has a cross sectional shape in the form of a circular segment whose radius and rise is selected in accordance with the dielectric loading of the helix desired. The internal bore of glass tube 12 herein is defined as the diameter of a cylinder which touches the innermost portion of each flute simultaneously and is selected to be slightly larger than the external diameter of helix 10 to permit insertion thereof. Since the rise (radial thickness) of the flutes depends on the permissible loading of helix 10, the internal diameter of tube 12 from which the flutes depend inwardly is selected by adding the radius of the bore and the rise of the flutes.

By way of example, the following materials and dimensions of helix 10 and glass tube 12 have been found eminently useful in practicing the instant invention. Helix 10 may be wound of Tungsten wire having a size of v006 inch with a pitch of .012 inch and an overall length of approximately 9.5 inches. Glass tube 12 is commercially available from Wilmad Glass Co., Buena, N.J., as Wilmad #7720 Nonex Glass Envelope. This particular glass tube has an outside diameter of .090 inch, a bore of .0547 inch and an inner diameter of .060 inch so that the rise of each flute is .0177 inch. The radius of curvature of each flute is .025 inch and the flutes are displaced 120 degrees with respect to one another.

This tube is ideally suited for accommodating a helix having an outside diameter of .0543 inch. In fact, in practicing the instant invention a radial clearance between the helix and the bore of the tube is .00015 to .00025 which has been found to give excellent results.

The method of bonding helix 10 to glass tube 12 will now be described. Helix 10 is inserted into glass tube 12 and placed until it assumes the desired lateral position with respect to bulb 14. The ends of helix 10 are then affixed to glass tube 12 by means of a pair of plate tabs or clamps 30 and 32. Thereafter both ends of glass tube 12 are hermetically sealed at 34 and 36 respectively and the sealed tube is evacuated by suitable pumping means until a negative pressure between 10- and 10* mm. of mercury has been established. The negative pressure is not critical and is selected in accordance with the inward pull desired and the time taken to reach the negative pressure. After the desired negative pressure has been established, the bonding operation may be commenced.

The bonding operation comprises the progressive heating of a single flute of glass tube 12 to its softening point so that it moves radially inward under the force exerted thereon by the negative pressure to touch the opposite helix portion. As soon as the inwardly moving flute touches the coils of the helix, a metal-to-glass seal or bond is formed which is often described as wetting. The degree of inward displacement of the heated flute depends of course on its softness since this determines how fast it moves inwardly for a given applied force; the length of time it remains soft since this determines the time over which the flute moves inwardly; and on the negative pressure since this determines the magnitude of the applied force.

The flame of the torch is preferably concentrated over a small circumferential sector of the envelope so that the bond is narrow. The narrowness in radians of course depends on the width of the flute, but generally speaking it should be substantially less than 1.0 radian.

Referring now to FIG. 4 there is shown diagrammatically an apparatus 40 useful in carrying out the bonding operation. Apparatus 40 comprises a table 41 supporting a pair of rotary mounted axially aligned chucks 42 and 44- which are adapted to clamp opposite ends of glass tube 12. There is also provided a torch 46 connected, by means of a flexible hose 48, to a pressurized hydrogen supply 50 and a pressurized oxygen supply 52. Torch 46 is held in a bracket 54 which is mounted to a movable torch carriage 56. Carriage 56 is motorized and travels at an adjustable rate of speed parallel to the chuck axes.

In practicing this invention, glass tube 12 is rotated in chucks 42 and 44 untilthe bisector of a selected flute, such as 25, is aligned with torch 46. Torch 46 is then moved to one end of tube 12 and ignited to provide a flame 58. The hotness and position of flame 58 is then adjusted, as will explained, to soften flute 26 as torch carrier moves flame 58 along tube 12. As flute 26 is urged inwardly a desired amount it moves into helix 10 as best seen in FIG. 333 to wet the helix.

Generally speaking, since the dielectric loading of helix 10 is usually desired to be kept at a minimum, it has been found that as little penetration of flute 26 into helix 10 as possible produces the most satisfactory results. Actual measurements have shown that a penetration of between .0002 and .0005 inch provides minimum loading and a permament bond.

For the #7720 glass described above the softening point is approximately 750 C. Since flame 58 requires to be hotter than 750 C. it has been found that the rate of softening and thereby the penetration of flute 26 into helix 10 is best controlled by adjusting the proximity and the rate at which torch 46 travels along envelope 12. For example if by means of an initial test piece it is deter mined that for a particular rate of motion, flute 26 penetrates too far into helix 10, as determined by phase velocity measurements, the rate of motion is increased until the proper penetration is obtained. As a matter of con venience, torch bracket 54 is provided with a vertical adjustment means 60 and a horizontal adjustment means; 62 so that the tip of flame 58 may be properly positioned with respect to tube 12.

Torch 46 may comprise a glass tube having a bore of .003 inch which is supplied with a mixture of hydrogen and oxygen which by volume is approximately 40% to 60% and a typical rate of motion of torch carriage 56 may be in the neighborhood of 1 /2 to 2 inches per minute.

After bonding, glass tube 12 is removed from chucks 42 and 44 and opened at both ends so that the tube may be assembled in the conventional manner such as in-' serting the electron gun means and sealing the tube again- With the leads in place.

There has been described hereinabove a means anda method for bonding a helix to a glass tube which provides minimum dielectric loading on the helix and which? is simple and economical. Even though the invention has been explained in connection with an inwardly fluted glass envelope, the method for bonding hereinabove described is equally applicable to bonding a helix to a straight cylindrical bore if the resulting loading is found unobjectionable.

What is claimed is:

1. The method of bonding a traveling wave tube helix to a traveling wave tube envelope comprising the steps of:

(a) positioning an internally unsupported helix into said envelope;

(b) sealing the ends of said envelope to provide a sealed enclosure;

(c) reducing the pressure in said sealed enclosure below atmospheric pressure; and

(d) concentratively heating a narrow longitudinal segment of said envelope to its softening point so that said segment moves inwardly under the force ap-' plied to it by reduced pressure in said sealed enclosure for bonding contact with said helix. 2. The method of bonding a traveling wave tube helix to a traveling wave tube envelope comprising the steps of: (a) positioning an internally unsupported helix into said envelope so that its axial position corresponds to the desired position; (b) sealing the ends of said envelope to provide a sealed enclosure; (0) reducing the pressure in said sealed enclosure below atmospheric pressure; and (d) concentratively heating a narrow longitudinal segment of the wall of said envelope to its softening point so that said segment moves inwardly under the force applied to it by the reduced pressure in said sealed enclosure for bonding contact with said helix. 3. The method of bonding a traveling wave tube helix to a traveling wave tube envelope comprising the steps of: (a) positioning an internally unsupported helix into said envelope so that its axial position corresponds to the desired position; (b) sealing the ends of said envelope to provide a sealed enclosure; (0) reducing the pressure in said sealed enclosure below atmospheric pressure; and (d) concentratively heating a narrow longitudinal segment of the wall of said envelope until it is soft so that said segment moves inwardly under the force applied to it by reduced pressure in said sealed enclosure and maintaining said heat until said segment has penetrated into said helix a selected moment. 4. A method in accordance with claim 3 in which said envelope has a plurality of inwardly extending flutes and in which the heated narrow longitudinal segment encompasses one of the flutes.

5. The method of bonding a traveling wave tube helix to a traveling wave tube envelope comprising the steps of: (a) positioning an internally unsupported helix into said envelope so that is axial position corresponds to the desired position; (b) sealing the ends of said envelope to provide a sealed enclosure; (0) reducing the pressure in said sealed enclosure below atmospheric pressure; and (d) progressively and concentratively heating a longitudinal segment of the wall of said envelope of less than 1 radian arc length along its axial length to progressively soften the same so that the negative pressure within said envelope moves said softened segment progressively inward into contact with a corresponding segment of said helix to establish a bond therebetween. 6. The method of bonding a traveling wave tube helix to a traveling wave tube envelope comprising the steps of: (a) positioning an internally unsupported helix into said envelope so that its axial position corresponds to the desired position; (b) sealing the ends of said envelope to provide a sealed enclosure; (c) reducing the pressure in said sealed enclosure below atmospheric pressure; and (d) progressively moving the concentrated flame from a torch over a narrow longitudinal segment of the surface of said envelope parallel to the envelope axis to heat a circumferential segment of the wall of said envelope from one end to the other to progressively soften the wall, the proximity of the flame to the wall of said envelope and its rate of motion being selected so that the degree and the duration of softness allows the negative pressure in said envelope to move said segment inwardly into a corresponding portion of said helix for a selected amount of penetration for the establishment of a bond therebe-tween.

7. The method of bonding a segment of the wire helix of a traveling wave tube device to a corresponding seg ment of the tubular envelope comprising the steps of:

(a) selecting a tubular envelope having a bore diameter which is 0.0001 to 0.0005 inch larger than the external helix diameter;

(b) inserting an internally unsupported helix into the tubular envelope;

(c) sealing the tubular envelope and evacuating the same to a negative pressure lower than 10 mm. of Hg; and

(d) applying suflicient concentrated heat to an external narrow longitudinal wall segment of said tubular envelope to soften the wall segment sufficiently to move inwardly under the negative pressure into contact with the corresponding longitudinal segment of the helix.

8. The method of bonding a segment of the wire helix of a traveling wave tube device to a corresponding segment of the tubular envelope comprising the steps of:

(a) selecting a tubular envelope having a bore diam eter which is between 0.0001 and 0.0005 inch larger than the external diameter of the helix;

(b) inserting an internally unsupported helix into the tubular envelope and clamping the ends of said helix in place;

(0) sealing the tubular envelope and evacuating the same to a negative pressure lower than 10- mm. of Hg; and

(d) progressively and longitudinally applying sufiicient heat to an external longitudinal wall segment of said tubular envelope to soften a narrow wall section so that the same moves inwardly under the negative pressure, the amount of heat applied and the rate of progressive application of the heat being selected to allow the inwardly moving softened wall segment to penetrate into the corresponding segment of the helix a selected amount.

9. The method of bonding a segment of the wire helix of a traveling wave tube device to a corresponding segment of the tubular and inwardly fluted envelope comprising the steps of:

(a) selecting an envelope having a bore diameter which is between 0.0001 and 0.0005 inch larger than the external diameter of the helix;

(b) inserting an internally unsupported helix into the envelope and clamping the ends of said helix in place;

(c) sealing the envelope and evacuating the same to a negative pressure lower than 10 mm. of Hg;

(d) progressively applying a highly concentrated source of heat to an external longitudinal section of said tubular envelope which is radially aligned with one selected flute to soften the wall section sufficiently so that the same moves inwardly under the negative pressure, the amount of heat applied and the rate at which the source is progressed being selected to allow the inwardly moving flute to penetrate into the corresponding segment of the helix of not less than .00015 inch; and

(e) unsealing said envelope for the further assembling and fabrication of said traveling wave tube device.

References Cited by the Examiner UNITED STATES PATENTS 2,713,196 7/1955 Brown 29-4955 2,845,690 8/1958 Harrison 3l53.5 2,879,436 3/1959 Geisler 3153.5 3,119,043 1/1964 Karol 315-3.5 3,211,947 10/1965 Bloom 3153.5

JOHN F. CAMPBELL, Primary Examiner.

L. J. WESTFALL, Assistant Examiner. 

1. THE METHOD OF BONDING A TRAVELING WAVE TUBE HELIX TO A TRAVELING WAVE TUBE ENVELOPE COMPRISING THE STEPS OF: (A) POSITIONING AN INTERNALLY UNSUPPORTED HELIX INTO SAID ENVELOPE; (B) SEALING THE ENDS OF SAID ENVELOPE TO PROVIDE A SEALED ENCLOSURE; (C) REDUCING THE PRESSURE IN SAID SEALED ENCLOSURE BELOW ATMOSPHERIC PRESSURE; AND (D) CONCENTRATIVELY HEATING A NARROW LONGITUDINAL SEGMENT OF SAID ENVELOPE TO ITS SOFTENING POINT SO THAT SAID SEGMENT MOVES INWARDLY UNDER THE FORCE APPLIED TO IT BY REDUCED PRESSURE IN SAID SEALED ENCLOSURE FOR BONDING CONTACT WITH SAID HELIX. 